System for distribution of electric power

ABSTRACT

A subsea electrical power distribution system including a first chamber ( 20 ) housing a transformer, a second chamber ( 30 ) housing a power distribution system with busbars ( 32 ) for distribution of power to individual consumers connected to said busbars via connectors ( 33 ). The interface between the first and second chamber includes a flexible membrane ( 22 ) allowing the oil in the first chamber to expand into the second chamber and vice versa. The second chamber being connected to an external volume expansion chamber ( 34 ) allowing the pressure in the second chamber to balance against the pressure of the ambient sea.

FIELD OF THE INVENTION

The present invention relates to a system for distribution of electricpower between a transmission system and local consumers, especially in asubsea environment.

TECHNICAL BACKGROUND

In said system the transformer is the electrical link between the highvoltage transmission system and the local distribution system. Theoutput from the transformer is connected to a separate distribution orswitchboard unit connecting the transformer to the user equipment.

Transformers for subsea installations customary consist of a chamberhousing the transformer proper with core and copper windings. Thischamber is filled with transformer oil. To balance the internal pressureof the chamber to the ambient seawater pressure, the transformerassembly is equipped with a volume compensator in the form of anexternal expansion chamber.

This compensator constitutes a barrier against the surrounding waterthat is important for the correct operation of the transformer. Waterleaking into the transformer chamber can cause electric shorts betweenthe windings, which damages the transformer. The oil has very littleability to absorb contaminant water. To maintain this barrier intact isa problem for the manufacturer as the unit experiences a great span ofpressures from the surface down to the sea bottom. An eventual leak willalso pose a pollution problem.

The distribution unit is contained in its own oil filled housing withpressure compensation mechanism, separate from the transformer housing.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improvedarrangement of the transformer and the distribution unit in a system fordistribution of electric power, especially in a submarine environment.

This is achieved by a system including a transformer arranged in a firstchamber, said first chamber being filled with an insulating medium, anda distribution unit arranged in a second chamber, said distribution unitbeing connected to the transformer and comprising connections forconnection of the distribution system to the consumers, said secondchamber being filled with an insulating medium, wherein the transformerand the distribution unit are combined into a single module, theinsulating medium in the first chamber being separated from theinsulating medium in the second chamber, and the module comprises afirst volume compensation device for equalising the pressure between theinsulating medium in the first chamber and the insulating medium in thesecond chamber, and a second volume compensation device for equalisingthe pressure between the insulating medium in the second chamber and anambient medium surrounding the module.

One advantage of integrating the transformer and the distribution unitinto one module is that there are fewer electrical connections exposedto the seawater or any other ambient medium, and thus the reliability isincreased. Integrating the distribution unit and the transformer intothe same subsea module results in fewer interfaces exposed to theambient medium.

Another advantage of the present invention is that the second chamberwill serve as an additional barrier for the transformer. This additionalbarrier will eliminate the need for a double tank design of thetransformer housing.

The integration of the distribution unit and the transformer into onesingle module will also reduce the number of control/monitoringinterfaces. This integration will also imply that a reduced number ofseparate units have to be installed subsea and that a reduced number ofelectric connections have to be made up subsea.

Preferred embodiments as well as advantageous features of the inventionwill appear from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to the appendeddrawings, in which:

FIG. 1 shows an arrangement of a transformer unit with volume expansiontank and a separate distribution unit, according to prior art.

FIG. 2 shows a schematical cross section of a power distribution systemaccording to an embodiment of the present invention.

FIG. 3 shows a very schematical illustration of a volume compensationdevice included in a preferred embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1 gives an overview of a subsea power distribution system accordingto prior art. In said system the transformer 1 is the electrical linkbetween a high voltage transmission system and a local distributionsystem. The transmission voltage, typically 11 kV-36 kV, is transformeddown to a distribution level, typically 3.0 kV-12.0 kV. The traditionaldesign of a transformer is to have a single three-phase primaryconnection 11 and a single three-phase secondary connection 12. Theprimary input is connected to the transmission line via a high voltageconnector 4 mounted on the top of the transformer housing 9.

The secondary output 12 from the transformer is connected to a separatedistribution or switchboard unit 5. The distribution components in saidunit include a busbar arrangement with connectors and switchesconnecting the transformer to the consumer equipment. There is onebusbar 6 per phase. The consumers are connected to the busbars 6 viaconnectors 7 mounted on the wall of the housing.

The transformer unit 1 comprises a chamber 8 housing the transformercore and windings. The transformer chamber 8 is filled with transformeroil. The oil serve to cool the transformer by transferring the heatdeveloped in the transformer windings to the outer wall 9. Said wall 9is equipped with ribs on the outside to promote the transfer of heatinto the surrounding water mass.

To balance the internal pressure of the chamber 8 to the ambientseawater pressure, the transformer unit 1 is equipped with a volumecompensator in the form of an external expansion chamber 10. Theexpansion chamber 10 is in fluid communication with the oil inside thetransformer chamber 8, and in fluid communication with the externalwater via a membrane or/and a piston.

This oil filled chamber 8 constitutes a barrier, which protects thetransformer against the ambient seawater.

FIG. 2 shows a system for distribution of electric power between atransmission system and local consumers, especially in a subseaenvironment, according to an embodiment of the present invention. Here,the transformer and distribution unit are combined into a single module40. The module 40 includes a first chamber 20 containing the transformerwith its core and windings. This first chamber 20 is filled with aninsulating medium, preferably in the form of transformer oil. The module40 also includes a second chamber 30 containing the distribution unit,said unit being connected to the transformer and comprising connections33 for connection of the system to the consumers. The second chamber 30is also filled with an insulating medium, preferably in the form of oil.The second chamber 30 is suitably mounted above the first chamber 20, asillustrated in FIG. 2. However, the second chamber 30 could, if sodesired, be located at the side of or below the first chamber 20. Thefirst chamber 20 and the second chamber 30 are preferably arrangeddirectly adjacent to each other separated by an intermediate wall 15, asillustrated in FIG. 2.

The insulating medium in the first chamber 20 is separated from theinsulating medium in the second chamber 30 so as not to allow the mediain the two chambers to mix with each other. The module 40 has a two stepvolume compensation system comprising at least two different volumecompensation devices. The module 40 comprises a first volumecompensation device for equalizing the pressure between the insulatingmedium in the first chamber 20 and the insulating medium in the secondchamber 30, and a second volume compensation device for equalizing thepressure between the insulating medium in the second chamber 30 and anambient medium, such as seawater, surrounding the module 40.

In the embodiment illustrated in FIG. 2, the first volume compensationdevice 22 comprises a flexible membrane, preferably of rubber, arrangedin the interface between the first and second chambers 20, 30, i.e. herein the wall separating the two chambers. The flexible membrane 22 allowsthe insulating medium of the first chamber 20 to expand into the secondchamber 30 and vice versa. The flexible membrane will consequentlycompensate for expansion/contraction of the insulating medium in thefirst chamber 20 into or out of the second chamber 30. The first volumecompensation device could as well comprise several separate flexiblemembranes arranged in the interface between the first and secondchambers 20, 30.

In the embodiment illustrated in FIG. 2, the second volume compensationdevice comprises an expansion chamber. The insulating medium in thesecond chamber 30 is compensated to the ambient medium, i.e. the mediumsurrounding the module 40, by this expansion chamber. According to apreferred embodiment of the invention, the expansion chamber 34 includesa flexible membrane and/or a piston, the expansion chamber 34 being influid communication with the second chamber 30 on a first side of saidmembrane/piston, and in fluid communication with the ambient medium onthe other side of said membrane/piston. Said membrane is suitable formedas a bladder. The second volume compensation device could as wellcomprise several separate expansion chambers.

According to a particularly preferred embodiment of the invention, thesecond volume compensation device comprises two series connectedexpansion chambers, as schematically illustrated in FIG. 3.

The external, second volume compensator is the weakest link in regard tokeeping ambient seawater out of the system. The external compensator 34is attached to the second chamber 30, even if the volume of theinsulating medium in the first chamber 20 is greater than the volume ofthe insulating medium in the second chamber 30. This will require abigger internal membrane between the two chambers, but will also createan extra barrier for the transformer towards the ambient seawater.Consequently, ambient seawater that, for some reason, might leak intothe module 40 via the external compensator will by the internal, firstvolume compensation device be prevented from reaching the transformer.In this way, the transformer will in a very efficient manner beprotected from the ambient seawater.

The module 40 is provided with a high voltage connection 31 forconnection of the transformer to the transmission voltage of atransmission system. In the embodiment illustrated in FIG. 2, said highvoltage connection 31 is arranged on an outer wall of the second chamber30. As previously mentioned, the module is also provided withconnections 33 for connection to the consumers. In the embodimentillustrated in FIG. 2, the last mentioned connections 33 are alsoarranged on an outer wall of the second chamber 30. The distributionunit includes a number of busbars 32, one for each phase of thesecondary windings of the transformer, each busbar 32 being connected tothe associated secondary winding and to the connections 33. In theembodiment illustrated in FIG. 2, each busbar 32 is connected to theassociated secondary winding of the transformer via a conductor 35penetrating the wall between the first 20 and second 30 chamber.

In the embodiment illustrated in FIG. 2, the transmission voltage isentering the upper chamber through the high voltage connection 31,continues through the upper, second chamber 30, penetrates the top coverof the lower, first chamber 20 and connects to the primary windings ofthe transformer. The secondary output from the transformer penetratesthe top cover of the lower, first chamber 20 via bushings 21, and isconnected to the individual busbars 32 in the upper, second chamber 30.Distribution to the consumers is via a number of external connections 33coupled to the busbars 32. The upper, second chamber 30 may contain amore complex distribution unit than here described, including switchesfor turning on/off the individual consumers etc.

The module 40 preferably comprises at least one sensor for monitoringtemperature and/or at least one sensor for monitoring pressure (notshown). A hydrogen sensor may be incorporated in the module 40 in orderto indicate partial discharges (PD). A high hydrogen concentration is anindication of high PD activity in the transformer, which often is causedby water in the insulating oil. The monitoring sensors suitablycommunicate directly with the subsea distribution unit, i.e. there areno electronic cards inside the first chamber 20.

The invention is of course not in any way resricted to the prefferedembodiments described above, but many possibilities to modificationsthereof will be apparent to a man with with ordinary skill in the artwithout departing from the basic idea of the invention such as definedin the appended claims.

1. A system for distribution of electric power between a transmissionsystem and local consumers, the system comprising: a first chamberfilled with an insulating medium; a second chamber filled with aninsulating medium, wherein the first chamber and the second chamber arearranged adjacent to each other and the insulating medium in the firstchamber is separated from the insulating medium in the second chamber;an intermediate wall separating the first chamber from the secondchamber; a transformer arranged in the first chamber; a distributionunit arranged in the second chamber, said distribution unit beingconnected to the transformer and comprising connections for connectionof the distribution system to the consumers, wherein the transformer andthe distribution unit are integrated into a single module, the modulecomprising a first volume compensation device for equalizing thepressure between the insulating medium in the first chamber and theinsulating medium in the second chamber, and a second volumecompensation device for equalizing the pressure between the insulatingmedium in the second chamber and an ambient medium surrounding themodule.
 2. The system according to claim 1, wherein the first volumecompensation device comprises a flexible membrane, arranged in theintermediate wall between the first and second chambers.
 3. The systemaccording to claim 2, wherein the flexible membrane is arranged in saidintermediate wall.
 4. The system according to claim 2, wherein theflexible membrane is arranged to allow the insulating medium of thefirst chamber to expand into the second chamber.
 5. The system accordingto claim 2, wherein the flexible membrane comprises rubber.
 6. Thesystem according to claim 1, wherein the second volume compensationdevice comprises an expansion chamber.
 7. The system according to claim6, wherein the expansion chamber includes a flexible membrane, theexpansion chamber being in fluid communication with the second chamberon a first side of said membrane, and in fluid communication with theambient medium on the other side of said membrane.
 8. The systemaccording to claim 6, wherein the second volume compensation devicecomprises two series connected expansion chambers.
 9. The systemaccording to claim 6, wherein the expansion chamber includes a flexiblemembrane and a piston, the expansion chamber being in fluidcommunication with the second chamber on a first side of saidmembrane/piston, and in fluid communication with the ambient medium onthe other side of said membrane/piston.
 10. The system according toclaim 6, wherein the expansion chamber includes a piston, the expansionchamber being in fluid communication with the second chamber on a firstside of said piston, and in fluid communication with the ambient mediumon the other side of said piston.
 11. The system according to claim 1,wherein the distribution unit includes a number of busbars, one for eachphase of the secondary windings of the transformer, each busbar beingconnected to the associated secondary winding via a conductorpenetrating the wall between the first and second chamber, each busbarbeing connected to connections arranged on an outer wall of the secondchamber.
 12. The system according to claim 1, wherein the insulatingmedium in the first chamber is transformer oil.
 13. The system accordingto claim 1, wherein an insulating medium in the second chamber is oil.14. The system according to claim 1, wherein the second chamber ismounted above the first chamber.
 15. The system according to claim 1,wherein a high-voltage connection for connection of the module to atransmission system is arranged on a wall of the second chamber, theprimary windings of the transformer being connected to said high-voltageconnection via a conductor extending through the second chamber andpenetrating the wall between the first and second chamber.
 16. Thesystem according to claim 1, wherein the module is provided with asensor for monitoring temperature.
 17. The system according to claim 1,wherein the module is provided with a sensor for monitoring pressure.18. The system according to claim 1, wherein the module is provided witha hydrogen sensor for detecting partial discharges in the transformer.19. The according to claim 1, wherein the environment is a subseaenvironment.